This is a continuation of application Ser. No. 656,152, filed on Sept. 28, 1984, now abandoned.
In certain applications, it is desirable to drive two independent load devices with separate series wound DC motors which are connected in a series circuit and energized from a common voltage source. A particular example is the motive power system for an industrial truck which gave rise to the discovery and invention set forth herein. It has been the practice in certain industrial trucks to use two separate series motors for driving the two traction wheels. The reason for using two separate motors instead of one larger motor is cost; the power train using a single large motor costs considerably more than separate drive trains using two smaller motors. Heretofore, the two series motors have been connected electrically in parallel and each motor is provided with its own separate reversing contactor and certain other separate circuit components.
It has been recognized that additional cost savings could be realized if the two separate series wound drive motors could be connected electrically in series instead of parallel. In such an arrangement, a single reversing contactor and other circuit components could be used in common for both motors. However, the connection of DC series motors in series with each other has heretofore required special control circuits which add considerable cost and complexity and result in reduced reliability of the system.
According to conventional wisdom, DC series motors connected in series with each other cannot be operated to drive independent loads without some form of special control circuit to control the voltage across the respective motors. The reason behind this conventional thinking is as follows. If one motor has no load or light load and the other motor has a heavy load, the motor without load will have greater acceleration and will reach a high speed before the heavily loaded motor starts. The motors develop a counter electromotive force (EMF) as a direct function of speed. The speed of the lightly loaded motor increases quickly to the point where the counter EMF produced thereby is substantially equal to the applied voltage while the loaded motor remains at standstill and develops no counter EMF. The applied voltage is said to be equal to the sum of the counter EMF and the IR drop, i.e. the voltage drop produced by the current through the series resistances of the armatures and field windings of the motors. The torque produced by a series motor is proportional to the product of the field current and the armature current. The effect of increasing counter EMF due to the increasing speed of the unloaded motor is to reduce the current through the fields and armatures of the two motors so that very little torque is produced by the motors. As a result, the heavily loaded motor remains at standstill while the unloaded motor spins at high speed. In the case of a vehicle drive using series connected series motors for independent traction wheels, the vehicle would remain at standstill if one traction wheel is on ice or is off the ground while the other traction wheel is on the ground.
In the prior art, the problem of series motors in series connection with independent loads has been solved by providing special control circuits for controlling the voltage across the respective motors. A typical example of such prior art is the Cronberger U.S. Pat. No. 2,930,957 which issued Mar. 29, 1960. The system of the Cronberger patent utilizes speed sensing or voltage sensing means for the respective motors and when one motor runs faster than the other the field current of that motor is shunted to allow the speeds to equalize.
A general object of this invention is to overcome certain disadvantages of the prior art.